Study Of Gas Phase Reaction Mechanism Of AIN Grown By MOVPE

AlN is an important third generation semiconductor material with wide direct band gap,radiation resistance,high temperature resistance and high breakdown field strength.It can be used to fabricate laser diode,high brightness LED,UV photovoltaic device and so on.Organometallic vapor phase epitaxy?MOVPE?is the most widely used technique for the growth of AlN thin films,which is suitable for the growth of large area films,and the composition and thickness of the films can be accurately controlled.In this method,mainly by carrier gas?H₂,N₂?,the metal organic gas?TMAl?and ammonia?NH₃?are carried into the reaction chamber.The complex chemical reaction takes place in the reaction chamber.Finally,AlN thin films are grown on the high temperature substrates.In the process of AlN-MOVPE growth,since the source gas TMAl reacts with NH₃ to form a strong coordination bond,the growth rate of AlN is low,the gas consumption of the reaction source is large,and the nanoparticles generate.All of these are related to the chemical reaction in the reactor.The mechanism of gas phase reaction of AlN,especially the adduct path between TMAl and NH₃ at different temperatures and the influence of carrier gas on the gas phase reaction path,is still not well understood.Based on the theory of density functional theory and the transition state theory,the adduct reaction path and the hydrogenolysis path of AlN were studied and analyzed.The possibility of each reaction path was studied by calculating and comparing the enthalpy and Gibbs free energy at different temperatures.The main contents are as follows:?1?Based on previous studies on the mechanism of gas-phase chemical reaction of AlN-MOVPE growth,the main adduct reaction path of growth was summarized,including the formation of amide,the bimolecular collision reaction with NH₃,the formation of dimer and trimer and the subsequent CH₄ elimination reaction.?2?The adduct path of AlN-MOVPE growth process was calculated.It was found that the adduct TMAl:NH₃ were formed immediately after the mixing of TMAl and NH₃ at room temperature.At higher temperature,the adducts may be redecomposed and may undergo a transition state.After overcoming the energy barrier of about 27 kcal/mol,a methane is removed to form the amide DMAlNH₂.At a higher temperature,TMAl can react directly with NH₃ and can easily cross the lower barrier??H?5⁸ kcal/mol?to form the amide DMAlNH₂.The amide can eliminate two methane successively through two bimolecular collisions with NH₃ to form the stable Al?NH₂?₃.Amide can also be polymerized into dimer or trimer,and then gradually eliminate methane.In the temperature range of 641¹111 K,it will be advantageous for the dimer to eliminate the methane to form the?MMAlNH?₂.In the temperature range of 385⁶16 K,it will be advantageous for the trimer to eliminate CH₄ to form?MMAlNH?₃.However,The reaction of?MMAlNH?₂ and?MMAlNH?₃continuing to eliminate methane to form?AlN?₂ and?AlN?₃ are difficult to take place.Because the Gibbs free energy difference is greater than zero.The results show that Al?NH₂?₃,?MMAlNH?₂ and?MMAlNH?₃ are the most likely three kinds of gas phase reaction precursors,which will determine the surface reaction growth of AlN.?3?The hydrogenolysis path of AlN-MOVPE growth process was calculated.It was found that the source gas TMAl could collide with the carrier gas H₂,and methane was removed successively,and finally AlH₃ was formed.The energy barrier required for each step is about 27 kcal/mol.When T?885 K,the adduct reaction of AlH₃ and NH₃ can take place to form the adduct AlH₃:NH₃.At higher temperature,AlH₃:NH₃ may be redecomposed,or after overcoming the energy barrier of about26²8 kcal/mol removing hydrogen.AlH₃ can also produce a bimolecular reaction with NH₃.In the collision reaction,the hydrogen is removed step by step,and the Al?NH₂?₃ is finally produced,and the energy barrier required for each step is very small.AndthebondenergyrelationshipD[?CH₃?₂Al-CH₃]<D?H₂Al-H?<D[NH₂)₂Al-NH₂],so the Al?NH₂?₃ is the most stable of the three substances.The results show that H₂ can react with TMAl,and the generated AlH₃ can continue to react with NH₃ to form a stable gas reaction precursor Al?NH₂?₃.